PROJECT SUMMARY Catheter ablation of cardiac arrhythmias, typically performed with radiofrequency (RF) energy, is used in medically refractory cases, but is limited in efficacy by shallow depth of ablation and failure to selectively target persistently conducting myocardium amidst heterogeneous scar. Histotripsy is a novel ultrasound (US) ablation method that deeply focuses short ultrasound pulses with high amplitudes to mechanically disintegrate tissue, forming well-demarcated lesions. The presence of cavitation nuclei in the form of stable encapsulated microbubbles (MB) lowers the power necessary to produce tissue necrosis. The overall goal of this proposal is to evaluate whether histotripsy can generate large lesions with focused depth necessary for more effective ablation. We also hypothesize that intravascular MBs will further improve ablation efficacy by lowering histotripsy threshold specifically in areas of conducting viable myocardium that often interdigitate heterogeneously with scar. Aim 1 will use both ex-vivo myocardium and in-vivo swine models to assess the ability of non-contrast histotripsy to create large, deep, focused myocardial lesions. The high-throughput ex- vivo model will allow us to efficiently test the effects of varying the histotripsy acoustic parameters (peak negative pressure, pulse duration and frequency) and to simultaneously assess acoustic response responsible for ablation by passive cavitation detection (PCD). Best candidates will then be prospectively tested in-vivo. Aim 2 will define the benefit of intravenously-injected MBs in potentiating histotripsy using various US parameters in order to test whether cavitation threshold is reduced and lesions are larger and more homogenous compared with non-contrast histotripsy. Once ideal conditions are defined, then intravital microscopy with simultaneous PCD will be performed to define the biologic events that underly contrast- enhanced histotripsy real-time in an in-vivo rat cremaster muscle model. To further elucidate mechanism, we will study the effect of MB compositional changes (lipd shell and gas core) on the predisposition to cavitation. Aim 3 will use a swine myocardial infarction model to assess the ability of infused MBs to target ablation toward persistently perfused and conducting ?channels? within heterogeneous areas of scar, which represent the arrhythmogenic source in patients. These aims will build toward a paradigm shift in ablation whereby energy is targeted directly at pathologic yet persistently conducting myocardium that sustains arrhythmias. With this project, Dr. Nazer builds upon his background in biomedical engineering and therapeutic ultrasound research, and has defined a translational research topic that is compatible with his clinical work as a cardiac electrophysiologist. He is cognizant of the challenges faced by physician-scientists in procedurally- oriented fields, but has assembled the necessary protected research time, translational research focus, institutional commitment and resources, and mentorship and collaborator team to maximize his chance of succeeding as an independent translational physician-scientist.